In order to avoid the occurrence of trapped air in the finished cast work piece, the diecasting mould and the mould cavity of the diecasting mould, respectively, has to be vented during the diecasting operation. Thereby, it must be ensured that not only the air contained in the cavities of the diecasting machine and of the diecasting mould can escape, but it has also to be ensured that gases escaping from the liquid casting material can be removed as well.
In venting a diecasting mould, there is a problem insofar as the venting valve should be closed as late as possible in order to ensure that the mould cavity is vented, if possible, until it is completely filled; on the other hand, it should be avoided that liquid casting material can enter the venting valve. Taking this problem into account, venting assemblies have been disclosed in the prior art having a venting valve that is operationally connected to an impact transmitter operated by the liquid casting material advancing from the mould cavity into the venting channel. With the aid of such a design, very reliable valve assemblies can be realized that can be operated very quickly. In order to be able to build up a ram pressure at the impact transmitter sufficient to perform the closing operation, the venting channel comprises returns and cross sectional area variations. Moreover, the venting channel must have a certain minimal length between the impact transmitter and the real valve body member of the venting valve, and it should be full of corners in order to ensure that the venting valve is closed before the liquid casting material has reached the venting valve. By a design of the venting channel running between the impact transmitter and the venting valve that is full of corners, moreover, it can be avoided that splashes leading the real flow of casting material can enter the venting valve and block it. In order to increase the efficiency of such valve assemblies, usually a vacuum pump is connected to the venting valve.